1. Field of the Invention
This invention relates to a multi-level inverter wherein three or more voltage levels are varied gradually to effect pulse amplitude modulation (hereinafter referred to briefly as "PAM") control.
2. Description of the Prior Art
The multi-level inverter contemplates that three or more voltage levels are varied gradually to approximate stepped forms so as to effect PAM control. In addition, it is possible to effect pulse width modulation (hereinafter referred to briefly as "PWM") control between two voltage levels in the vicinity of the level changes. Because of this, the multi-level inverter can decrease the number of commutations and decrease harmonic frequency components, thus reducing switching losses. The extremely low harmonic frequency components in the current outputted from the multi-level inverter is advantageous in that, in driving an AC motor as a load, losses due to torque ripples and harmonic frequency contacts can be reduced to a considerable extent.
Heretofore, there have been published circuit arrangements of multi-level inverters of various types.. One example thereof is shown in FIG. 1.
A pair of reference DC power supplies E.sub.1 and E.sub.2 such as batteries are connected in cascade connection to the multi-level inverter shown in FIG. 1. A set of capacitors C.sub.1 and C.sub.2 are connected in cascade connection to the power supplies E.sub.1 and E.sub.2, through the diodes D.sub.1 and D.sub.2 respectively. One of connecting points of the capacitors C.sub.1 and C.sub.2 is selectively connected to a positive or a negative pole of DC power supplies through a change-over switch SW.sub.1. A load L is connected to a connecting point of the DC power sources E.sub.1 and E.sub.2 and a common connecting point of the change-over switch SW.sub.2. The change-over switch SW.sub.2 has a function of selectively connecting the load L to the positive or the negative pole of the group of capacitors. Here, the capacitors C.sub.1 and C.sub.2 have functions as being the reference DC power supplies, and the diodes D.sub.1 and D.sub.2 have functions as the charging paths for charging the respective capacitors. Voltages at multiple levels can be applied to the load L through the combination of changed-over polarities by the switches SW.sub.1 and SW.sub.2.
In the circuit shown in FIG. 1, operations in four modes are performed and output voltages are on four voltage levels. In a first mode, the switches are connected such that the load and the reference DC power supply E.sub.1 form a closed loop (with SW.sub.1 being at the lower side, and SW.sub.2 at the upper side,) and applied to the load L is voltage E.sub.1. Then, a closed loop is also formed by the reference DC power supplies E.sub.1, E.sub.2 and the capacitor C.sub.1 and the capacitor C.sub.1 is charged to (E.sub.1 +E.sub.2). A second mode is a condition, in which the switches are connected such that a closed loop is formed by the load, the reference DC power supply E.sub.1 and the capacitor C.sub.1, which has been charged to (E.sub.1 +E.sub.2), (with both SW.sub.1 and SW.sub.2 being at the upper side), and applied to the load L is a voltage of (2E.sub.1 +E.sub.2). A third and a fourth modes are conditions, in which the switches SW.sub.1 and SW.sub.2 are thrown into positions symmetrical with those in the first and the second modes. Namely, the capacitor C.sub.2 operates in the same manner as with the capacitor C.sub.1 in the first and the second modes, and output voltages come to be -E.sub.2 and (-2E.sub.2 -E.sub.1).
FIG. 2 shows another example of the conventional multi-level inverter. The example shown in FIG. 2 is of such an arrangement that two sets of pairs of DC power supplies (E.sub.1 and E.sub.2, E.sub.3 and E.sub.4) are provided, and these power supplies are selectively combined by switches to obtain a plurality of modes without requiring the use of capacitors. In this case also, there exist the first through fourth modes depending on the positions of the switches in the same manner as with the inverter shown in FIG. 1, and applied to the load are voltage of four types including (-E.sub.2 +E.sub.3), (E.sub.1 +E.sub.3), (E.sub.1 -E.sub.4) and (-E.sub.2 +E.sub.4).
Now, in the second and fourth modes, in which the DC power supplies and the capacitors thus charged are interconnected in series and the voltage (2E.sub.1 +E.sub.2) or (-2E.sub.2 -) is supplied to the load, in order for the arrangement shown in FIG. 1 to be able to neglect the lowering in the output voltage due to the discharge of the capacitors, it becomes necessary to use capacitors of high capacities, whereby the system becomes large-sized, thereby presenting problems in practical use.
On the other hand, when the DC power sources are batteries, the capacities of the batteries are divided as in the circuit shown in FIG. 2, and the batteries thus divided are used in lieu of the capacitors, so that the large-sized circuit arrangement can be avoided. However, there is the disadvantage that it is difficult to uniformly consume the respective batteries. Further, in the first and third modes, there are two batteries which are connected in series with opposite polarities, and the differential voltages are applied to the load, and hence, the batteries must perform the charging and discharging, respectively. In this instance, the output impedance is increased, thus resulting in lowered conversion efficiency. In maintenance, when the batteries are changed, the respective batteries must be individually charged because the batteries are dispersed in their positions. In replacing the batteries with new ones, the working efficiency is low, and moreover, there is the disadvantage that the batteries must be charged independently of one another because values of electric charges consumed are different from one battery to another when they are taken out.
Further, in the arrangements shown in FIGS. 1 and 2, such basic arrangements are adopted that there should necessarily be included mode change-over switches between the two sets of pairs of DC power supplies. For this, in any mode, it is required to use a multiplicity of electronic switches in the closed loop of current flowing through the load. Moreover, the number of electronic switches included in the closed loop is increased with the increase in number of the closed circuit cascade-connected. For this, due to the heat loss generated in the individual electronic switches, general loss in the inverter is increased, thus presenting the disadvantage of lowered conversion efficiency of the inverter.